1. Field of the Invention
The present invention relates to a method for simultaneous processing of geophysical data acquired at different times. Specifically, Measurement-While-drilling (MWD) and wireline resistivity data acquired at different times with tools having different resolution under different borehole conditions are simultaneously inverted to obtain parameters relating to the formation and the vicinity of the borehole.
2. Background of the Art
Various embodiments of electromagnetic, nuclear and acoustic measurements have been made for many years to determine geophysical properties of earth formations penetrated by a borehole. These measurements are usually displayed as a function of depth within the borehole at which they were measured forming a display known in the industry as a xe2x80x9clogxe2x80x9d of the borehole. The log of spontaneous potential of earth formations penetrated by a borehole was made in 1927 using a wireline device. In the following decades, borehole measurements using wireline devices were expanded to include nuclear and acoustic measurements, as well as more sophisticated electromagnetic measurements, to determine additional geophysical parameters of interest, and to also determine certain properties of the borehole.
Historically, measurements of formations have been made using wireline techniques in which sensors conveyed on a wireline are used to make various types of measurements from which formation and borehole properties are determined. For example, electromagnetic induction logs typically are measured by an instrument which includes a transmitter, through which a source of alternating current (AC) is conducted, and includes receivers positioned at spaced apart locations from the transmitter. The AC passing through the transmitter induces alternating electromagnetic fields in the earth formations surrounding the instrument. The alternating electromagnetic fields induce eddy currents within the earth formations. The eddy currents tend to flow in xe2x80x9cground loops,xe2x80x9d which are most commonly coaxial with the instrument. The magnitude of the eddy currents can be related to the electrical conductivity (the inverse of the resistivity) of the earth formations. The eddy currents generate alternating secondary magnetic fields which, in turn, induce voltages in the receivers which, generally speaking, are proportional to the magnitude of the eddy currents. Various circuits are provided in the instrument to measure the magnitude of the induced voltages, and thus determine the conductivity (and therefore the resistivity) of the earth formations. Such a method is disclosed in Beard et al. (U.S. Pat. No. 5,841,281) wherein an induction well logging is inserted into the wellbore. The instrument includes a transmitter, a source of alternating current connected to the transmitter, and a receiver. The transmitter is energized with the alternating current, voltages induced in the receiver are recorded, and area enclosed with respect to a baseline, by voltage peaks present in the recorded voltages is determined. The enclosed area corresponds to the conductivity.
Within the last few years, use of MWD tools for obtaining subsurface information has become more common. Meyer et al. (U.S. Pat. No. 5,469,062) discloses an invention directed toward the measure of geophysical parameters of earth formations penetrated by a borehole in an MWD environment. The invention employs propagation resistivity techniques utilizing a downhole instrument comprising multiple, longitudinally spaced transmitters operating at different frequencies with a plurality of longitudinally spaced receiver pairs. An electromagnetic wave is propagated from the transmitting antenna coil into the formation in the vicinity of the borehole and detected as it passes the receiving antenna of the receiver pair. The basic parameters measured at the receivers are the amplitude and phase shift of the sensed electromagnetic wave. The downhole instrument is conveyed along the borehole by a drill string or other means thereby making the basic measurements as a function of position or depth of the downhole instrument within the borehole. A plurality of parameters of interest can be determined by combining the basic measurements.
In the preferred embodiment disclosed by Meyer, both amplitude and phase measurements are made at two frequencies of 400 KHz and 2 MHZ and at two effective transmitter-receiver spacings. This yields a total of eight independent measurements per depth interval as the borehole device is conveyed along the borehole. In principle the set of eight equations can be solved for eight xe2x80x9cunknowns.xe2x80x9d The unknowns include the geophysical formation properties of resistivity and dielectric constant, and additional parameters quantifying invasion and borehole properties such as rugosity and ellipticity. The latter borehole parameters have been considered as sources of xe2x80x9cnoisexe2x80x9d in prior art resistivity measurements. When quantified, however, such borehole characteristics provide useful information concerning rock properties and the effectiveness of the drilling program.
MWD measurements are, as the name implies, made while a borehole is being drilled. The environment in which the sensors operate is much more hostile than the environment in which wireline devices operate. In addition, while a borehole is being drilled, there is less of a problem with xe2x80x9cinvasionxe2x80x9d than when making wireline measurements. Invasion refers to the degree to which the fluid within a borehole migrates into, or xe2x80x9cinvadesxe2x80x9d a formation surrounding the borehole. In an MWD environment, the measurements are typically made within a few minutes of the drilling of the borehole since the sensors are typically within a few feet of the drill bit, so that there is less likelihood of invasion. In contrast, wireline measurements are typically made several hours, days or even weeks after a borehole has been drilled. During this time interval, the borehole fluid may be changed and there is plenty of time for the borehole fluid to invade the formation. As a result of this, even if identical sensors were used to make the same measurements during drilling and days later on a wireline, the measurements made by the sensors will almost invariably be different because of invasion. The invaded zone typically has different electrical properties than the uninvaded formation. This makes a direct comparison of the measurements difficult. Furthermore, a comparison of the results of interpretation of the measurements made in two different epochs is even more problematic.
A second problem with evaluating measurements and interpretations made at different epochs using different tools arises from the differences in vertical resolution and depth of investigation of the tools that are used. Propagation resistivity tools of the type discussed in Meyer commonly operate at frequencies between 400 kHz and 2 MHZ. In contrast, induction logging tools commonly used in wireline applications operate at frequencies between 10 kHz and 200 kHz. The vertical resolution of a logging tool depends upon the wavelength of the interrogating signal: the shorter the wavelength, the finer the resolution. However, with increasing frequency, the depth of penetration decreases. A longer transmitter-receiver spacing, commonly referred to as a deep subarray, gives a greater depth of investigation, but gives rise to problems in resolving thin beds because the received signal includes effects of all intervening beds. A shorter transmitter-receiver spacing, commonly called a shallow subarray, has a smaller depth of penetration but may not have problems resolving thin beds. The shallower penetration means that a shallow subarray may not be able to accurately determine the depth of invasion of the formation by borehole fluids. For this reason, it is common to make measurements at different frequencies and different transmitter-receiver distances to get data with different resolution and depths of penetration.
Another level of complexity in the interpretation of well log resistivity measurements arises when the effects of anisotropy are considered. The prior art discussed above all assumed that the earth formations are isotropic. There have been numerous inventions that disclose the determination of anisotropic formation properties using induction logging tools. For example, Segesman et al. (U.S. Pat. No. 4,360,777) discloses an array of three individually energizable transmitter coils having mutually orthogonal axes. Electronic transmitter steering circuitry is provided for controlling the energizing means to electronically steer the direction of the magnetic moment resulting from the magnetic field components generated by the transmitter coils. An array of receiver coils is also provided along with receiver processing circuitry for processing signals induced in the receiver coils. The array of receiver coils preferably includes three receiver coils having mutually orthogonal axes. The receiver processing circuitry is capable of individually sensing the signals induced in the receiver coils and operates to combine the sensed signals. The receiver processing circuitry also includes electronic receiver steering circuitry for controlling the relative amplification of the sensed signals to steer the effective sensing direction of the receiver. The receiver steering circuitry is coordinated with the transmitter steering circuitry and is operative to rotate the effective sensing direction of the receiver in a plane perpendicular to the direction of the transmitter magnetic moment. Another example of a device for obtaining measurements of anisotropy is given in PCT publication WO 98/00733 on an application of Beard et al. With an arrangement of coils that are inclined to the borehole axis, the measurements may be processed to determine formation dip angles and anisotropic conductivities assuming a model of transverse isotropy. A transversely isotropic medium is characterized by a symmetry axis of infinite-fold rotational symmetry wherein the properties along the symmetry axis are different from properties in any direction in a plane orthogonal to the symmetry axis.
There is a need for a method of simultaneous processing and interpretation of well logs acquired at different epochs using tools of different resolution. Such an invention should preferably also be able to determine properties of anisotropic formations. The present invention satisfies this need.
The present invention is a method for joint inversion of data acquired at different epochs using different types of tools. In a preferred embodiment, a propagation resistivity tool is run first, preferably at several frequencies and several transmitter-receiver (T-R) spacings. At one or more later epochs, another tool run may be made, preferably on a wireline. The joint inversion process identifies bed boundaries based on inflection points in the propagation resistivity, induction logging, focused resistivity or other log data such as a gamma ray, density or an imaging tool. An initial guess for an uninvaded earth model is generated using the selected bed boundaries and the apparent raw resistivity values. An inversion run using shallow measurements of propagation resistivity and induction logging data is performed to estimate a resistivity structure representative of the near borehole zone resistivity (invaded zone). The bed boundary positions of the layers are also updated as part of the inversion process. Synthetic data for both the shallow and deep measurements are generated to delineate the invasion zones. If the synthetic data match from the shallow subarrays matches data from deep subarrays, then the model obtained from the shallow data is used as the final model for the inversion. In the event the data match is good for short subarrays and not for long subarrays, a final inversion run is performed by introducing invasion in the earth model.